1. Field of the Invention
The present invention concerns a method of exchanging data between a base transceiver station of a mobile radio network, for example a GSM (Global System for Mobile communications) network, and a mobile in the network.
2. Description of the Prior Art
A cellular mobile radio network includes a plurality of geographically dispersed base transceiver stations. Each base transceiver station provides radio coverage of a specific geographical area or cell. Each base transceiver station is usually at the center of the respective cell; the expression Base Transceiver Station is abbreviated to BTS.
Thus the overall coverage area is divided into cells and a mobile (such as a mobile telephone, for example) in the area covered by a network of base transceiver stations can at all times exchange data (usually speech) and signalling by radio with the base transceiver station of the cell in which it is currently located.
When a mobile user wishes to set up a call to another mobile user or a fixed user, i.e. to transmit and receive speech, the respective base transceiver station acts as a relay station and to this end has at least one transmitter and at least one receiver associated with at least one antenna.
The BTS antenna can be an omni-directional antenna transmitting high power signals in all directions.
However, signals received by a non-directional antenna are of poorer quality than signals received by a highly directional antenna. Consequently, the signals transmitted by the mobiles must be transmitted at a high level in order to be usable by a BTS having an omni-directional antenna.
The level at which mobiles transmit is limited, among other reasons to increase the length of time for which they can operate between battery recharging, with the result that if they are too far from the center of the cell, reception at the BTS is poor, the signal being at too low a level to be distinguished from interference or noise.
One method of overcoming this drawback is to divide the cells, especially large area cells (having a radius in the order of several kilometers), into a plurality of sectors grouped around the BTS of the original cell, referred to as the main cell. In this way a cell can be divided into three, six or 12 sectors, for example, depending on its size, the communication traffic in the area it covers and the power output of the mobiles from which it must be able to receive.
Each sector then has a directional antenna, all these directional antennas being located at the same site, usually at the center of the main cell. For convenience, the combination of all the sectors will continue to be referred to as a cell, although each sector in fact constitutes an independent cell in the sense as defined above.
Two main types of data, of the kind exchanged on the ISDN (Integrated Services Digital Network) can be exchanged between a mobile and the base transceiver station of the cell in which it is located:
speech (or computer) data, and PA1 signalling and control data, which includes information enabling the mobile to access the cellular network or to maintain a call in progress on changing from one cell to another ("Handover"). PA1 said mobile transmits at least one connection request or access request to said base transceiver station on a specific access channel on an upward control channel, PA1 a base station controller controlling said base transceiver station determines, for each of said receivers, the level and/or quality of the access signal constituted by said request and received at each of said receivers and in this way selects whichever of said receivers for which the access signal has the best quality and/or the best level as the receiver to be used for speech or computer data signals transmitted by said mobile, PA1 an upward traffic channel is assigned to said mobile for its exclusive use on said upward traffic frequency, PA1 a respective downward traffic channel is assigned to said mobile on said downward traffic frequency, said downward traffic channel being shifted in time relative to said upward traffic channel, and PA1 each of the other mobiles in said main cell and connected to said base transceiver station is associated with a downward traffic channel on said downward traffic frequency shifted in time relative to all the traffic channels assigned to the other mobiles on said downward traffic frequency and to an upward traffic channel on said upward traffic frequency shifted in time relative to all the traffic channels assigned to the other mobiles on said upward traffic frequency. PA1 during the time slot corresponding to the upward traffic channel assigned to said mobile each of said receivers, including the old receiver used for the speech or computer data signals transmitted by said mobile, receives the speech or computer data signals transmitted by said mobile, PA1 after demodulation of each of said speech or computer data signals, the signal obtained by combining said speech or computer data signals is shifted into the base band and transmitted to said base station controller, and PA1 communication continues on the same upward traffic channel via a new receiver. PA1 during the time slot corresponding to the upward traffic channel assigned to said mobile, each of said receivers, including the old receiver used for the speech or computer data signals transmitted by said mobile, receives the speech or computer data signals transmitted by said mobile, PA1 the speech or computer data signals from each of said receivers are shifted into the base band and then transmitted to said base station controller, PA1 said base station controller interrupts communication and, using one or more predetermined criteria, selects the receiver to be used for further communication and thus for said speech or computer data signals from said mobile to said base station controller, PA1 communication is established again on the same upward traffic channel, and PA1 the speech or computer data signals and signalling data signals from said mobile and from said selected receiver are shifted into the base band and then transmitted to said base station controller.
Thus each main cell is associated with a set of frequencies comprising at least: a frequency known as the downward control channel for transmitting general signalling data in the downward direction (i.e. in the direction from the base transceiver station to the mobile), the downward control channel being associated with a frequency known as the upward control channel for transmitting general signalling data in the upward direction (i.e. in the direction from the mobile to the base transceiver station). In GSM terminology control channels are called Broadcast Control Channels (BCCH).
Either channels not used for signalling on the upward and downward control channels or, if communication traffic in the cell is greater, at least one downward traffic frequency forming a frequency pair with an upward traffic frequency can be used to transmit speech or computer data.
In this case the frequencies of the upward and downward control channels and the upward and downward traffic frequencies are all different.
In the case of sectors of a large area cell with low traffic, an upward control channel and a downward control channel are associated with each sector and each sector is therefore provided with a transmitter and a receiver. These two frequencies can be sufficient in themselves to carry signalling and traffic in each sector.
Because GSM cellular networks employ Time-Division Multiple Access (TDMA), when a mobile accesses the cellular network it is assigned a traffic channel (TCH) corresponding to a transmit time slot reserved for it on the upward control channel (or on a specific upward traffic frequency if there is one). It is assigned a corresponding traffic channel for receiving on the downward control channel (or on the downward traffic frequency paired with the upward traffic frequency if there is one). The upward traffic channel is shifted in time relative to the downward traffic channel.
In a GSM network it is possible in practice to use eight channels per frequency. The time slots of the control channel not used for general signalling of the cell are also available for traffic and there are usually therefore seven traffic channels on the control channel in addition to the signalling channel.
In rural regions, where the cells usually cover a large area, and are therefore divided into sectors, many traffic channels are therefore available (since there is at least one pair of control frequencies per sector) and in practice very few are used because of the very low call density. The availability of at least one pair of control frequencies per sector requires each sector to be provided with a transmitter and a receiver.
Consequently, the equipment overhead is high for a low use of capacity, with the result that investment in a system of this kind is not cost effective.
Document WO 84/00654 describes a base transceiver station which solves this problem; it includes a single transmitter with an omni-directional antenna providing transmit radio coverage for all the sectors, so that the data transmitted by the base transceiver station by radio to a mobile in any of the sectors is invariably transmitted by this transmitter.
According to document WO 84/00654 the checks required when accessing a mobile station or on handover (i.e. when changing cell) are carried out by the base transceiver station itself. This makes the base transceiver station complex and costly. An object of the present invention is to remedy this problem.